The invention relates to a plasma torch with a transferred electric arc, a device for delivering the plasma gas in a vortex, thermal insulation on the outside of the torch, and an equal potential between the torch nozzle and the metal outer housing of the torch.
Plasma torches are realized by stabilizing an electric arc in a carrier gas. The simplest way to do this is to use a graphite electrode with an axial bore, through which the carrier gas flows into the electric arc that develops between the electrode and the material being melted. Along with these graphite torches, there are also cooled metal torches. They can be divided into two categories, torches with a non-transferred arc (indirect torches) and torches with a transferred arc (direct torches). In indirect torches, the electrode and counter electrode are integrated in the torch. In direct torches, an electrode is disposed in the torch, and the counter electrode represents the material to be treated. The introduction of the plasma gas is done either axially, thereby bathing a bar-like electrode, or tangentially into a gap that is located below a cooled hollow electrode. A gas vortex forms in a spiral in this hollow electrode. The bottom point of the arc is thereby moved over the inside surface of the electrode, and as a result the most uniform possible electrode abrasion takes place. The known embodiments are very vulnerable to malfunction and tend to form vertical electric arcs. This leads to rapid destruction of the electrode and to torch failure.
In addition, very high thermal losses occur. In some embodiments, the rotation of the arc is reinforced by auxiliary magnetic fields. These torches are available in an indirect embodiment (for instance from Union Carbide/Linde and Westinghouse) or in a direct version (for instance, Plasma Energy Corp., Retech).
In direct gas vortex plasma torches, the spacing between the electrode and the counter electrode in industrial application is generally not constant, since the material to be treated provides the counter electrode. This is true particularly for waste treatment, where the material to be treated is not distributed uniformly. When gas or dust is produced or when conductive layers develop because of dust deposits or condensation on torch parts, additional problems in operation arise. Disturbances in the gas vortex occur, with resultant local severe abrasion of the electrode that reduces its service life. Conductive layers lead to parasitic currents, which lead to secondary arcs that damage the torch. If the torch is accidentally extinguished during operation (for instance upon contact with a relatively large amount of nonconductive charge material), the local suction effect of the gas vortex can cause dust to be aspirated into the torch, which soils it and makes for deficient plasma gas supply; this makes further operation impossible, and the torch can be destroyed.